The invention is directed to a circuit for recovering the shape and spectrum of an optical signal after the signal has traversed a length of optical waveguide fiber. In particular, the circuit is used to preserve the shape of solitons over long lengths of waveguide fiber without use of electronic regeneration.
The value of soliton transmission of information over optical waveguide fibers is recognized in the art. The possibility of essentially dispersion-free transmission of pulses over long fiber lengths without electronic regeneration has encouraged work in the area of maintaining soliton signal integrity in extended transmission lines. With the introduction of waveguide fibers having attenuation in the range of a few tenths of a decibel per kilometer and optical amplifiers, solitons have become even more attractive as the transmission method of choice in very high bit rate systems or those that make use of wavelength division multiplexing.
A problem to be addressed in using soliton signals is how to control changes in the soliton time window, often called timing jitter, to avoid overlap with neighboring pulses. In addition, one should provide for filtering of noise that arises from the energy shedding of the soliton pulse as it undergoes shape changes during propagation along the fiber. Noise also originates in amplified spontaneous emission in the optical amplifiers
At bit rates  greater than 50 Gb/sec, the soliton pulse width must be less than about 5 ps to avoid overlap between adjacent soliton time windows. This pulse width is in general sufficient to minimize errors at the receiver end of the transmission. At the same time, the soliton period for these short pulse widths must be kept short relative to the preferred optical amplifier spacing, about 25 km. Thus there is a need to remove the energy lost by the soliton so that signal to noise ratios are at a desired level, inter-symbol interference is eliminated, and optical amplifiers do not become saturated by presence of noise signals.
Another consideration is the self-frequency shift of a narrow time width soliton due to differential Raman amplification of the pulse wavelength spectrum. This shift should be compensated in order to maintain the soliton wavelength within the desired low attenuation operating window and within the gain spectrum of the optical amplifiers.
The control of soliton timing jitter using a non-linear optical loop mirror (NOLM) is described in U.S. Pat. No. 5,757,529, Desurvire et al (""529). In the ""529 patent, a loop mirror is used as a switch that rejects system noise by preferential switching of the soliton signal through the NOLM. The switching is brought about my means of a stream of control pulses introduced into the NOLM. The overlap of the signal pulses and the control pulses in time determines the switching characteristics of the NOLM. Because relative timing difference between the signal pulse and control pulse are critical, the ""529 patent proposes a clock extraction or recovery circuit that produces a clock signal from the signal solitons. Such clock recovery circuits add considerable cost and complication to an optical circuit employing a NOLM. In some clock circuits, electo-optical devices are employed.
Thus, there is a need for a relatively simple and low cost means for recovering the shape of soliton signal after it has traversed a span of about 25 km of waveguide fiber. Also there is a need to address the problem of selffrequency shifting of the soliton signals without resort to elaborate, expensive optical or electo-optical circuits. The invention disclosed and described in this application incorporates simplicity and low cost into an optical circuit including a NOLM that simultaneously removes transmission circuit noise and recovers the original spectrum of the soliton signals.
The present invention is an optical circuit for noise filtering and frequency modulation of soliton pulses. The circuit includes a NOLM having its ends optically joined to the output ports of an NXN or first coupler, where N is at least two. Signal pulses are optically coupled to one of the input ports of the coupler and the signal pulses are thus divided into a clockwise (CW) and a counter-clockwise (CCW) stream of pulses propagating in the NOLM. A tap coupler is optically coupled to a point along the length of the NOLM. A stream of control pulses are input into the tap coupler which then couples the control pulses to the NOLM. Both the signal pulses and the control pulses can originate from a single optical soliton source. A source optically couples a stream of solitons to the input port of a splitting coupler which divides the stream of solitons into a stream of control pulses and a stream of signal pulses. Depending upon the direction along the loop mirror in which the tap coupler couples the control pulses, the control pulses will co-propagate with either the CW or CCW propagating signal pulses. The optical circuit is symmetric in the sense that the circuit may be configured to cause interaction between the control pulses and either the CW or CCW propagating signal pulses. It will therefore be understood that the description of the circuit given herein applies equally to the interaction of CW or CCW propagating signal pulses and the control pulses.
The optical path between the splitting coupler and the tap coupler is an optical fiber optically connected between the two couplers. So too, the optical path between the splitting coupler and the input of the first coupler is an optical fiber. The amount of interaction between the control pulses and the signal pulses depends upon the amount of overlap of the two sets of pulses as they travel through the optical loop mirror. This amount of overlap is controlled by selecting the lengths other two fibers that connect the respective pairs of coupler ports to provide a pre-selected lead or lag time of the signal pulses relative to the control pulses.
The interaction of control pulses with co-propagating signal pulses produces both a shift in centroid wavelength and phase (relative to the counterpropagating signal pulses) of the signal pulses. The centroid wavelength may be shifted up or down depending upon whether the control pulses lead or lag the co-propagating signal pulses. The amount of phase shift of the co-propagating signal pulse depends upon the magnitude of the lead or lag time between the control and signal pulses.
The selection of the lengths of the connecting waveguide fibers thus allows one to select the amount of centroid and phase shift of the co-propagating signal pulses in the NOLM.
In addition, the NOLM reflects the stray energy waveforms (noise), due to power shedding of the solitons or due to amplified spontaneous emission. Because the amplitude of the stray pulses is below the level at which nonlinear phase shifting occurs, no phase shift occurs between the CW and CCW propagating noise so they are not switched through the first coupler.
Thus the optical circuit disclosed and described herein serves to shift the centroid wavelength of the co-propagating signal pulses, shift the phase of the co-propagating signal pulse to switch the signal pulses thought the first coupler, and also to remove the low amplitude noise accumulated in the optical circuit. The shift in centroid can be chosen to offset any centroid shift of the solitons caused by differential Raman gain across the soliton spectrum. These functions are accomplished without use of clock extraction circuits or synchronized control pulses from a source separate from the signal soliton source.
In one embodiment of the optical circuit, the lead or lag time of the control pulses relative to the co-propagating signal pulses is within the range of about three times T. Here T is the soliton pulse width expressed as a time interval between the half maximum power points of the soliton.
An embodiment of the invention includes polarization adjusting components in one or both of the waveguides joined to the tap and first coupler. The control pulse is given a pre-selected polarization relative to the polarization of the signal pulse so that the control pulse may interact with the signal pulse and then be conveniently removed from the loop mirror fiber using a polarization selective coupler. The relative polarization of the respective control and signal pulses is such that the interaction between control pulses and the co-propagating signal pulses is effective to produce the desired phase shift and centroid shift of the signal pulses. Design alternatives that fall within the scope of this invention include relative polarization of control as compared to signal pulses that ranges from orthogonal to parallel. In those cases where the control and signal pulse axes are parallel or nearly so, other means of filtering the control pulse from the NOLM are available. Such means include gratings and wavelength or amplitude discriminators.
In an alternative embodiment, the tap coupler is polarization selective, to provide polarized control pulses in the optical loop mirror. After interaction with the co-propagating signal pulses, the control pulses may then be coupled out of the optical loop mirror by means of a second polarization selective tap coupler located farther along the optical loop mirror in the direction of travel of the control pulses.
In yet another embodiment of the invention, a band pass filter is inserted in the optical path, e.g., an optical fiber, of the signal pulses that have passed through the optical loop mirror. An advantageous feature of this configuration is that the center wavelength of the band pass filter can chosen to coincide with the centroid of the signal pulses exiting the NOLM. Because the NOLM as used herein shifts the centroid, a circuit can be designed which passes the centroid shifted solitons while rejecting noise near the wavelength of the original soliton. Thus the signal pulses pass through the band pass filter with minimum loss of power while noise pulses at the wavelength of the control pulses or original signal pulses are reflected or absorbed. This is an alternative use of the optical circuit as compared to use described above where the optical circuit including a NOLM is used to cancel self frequency shifting of the soliton due to differential Raman gain. Signal pulse centroid shifts in the range of +/xe2x88x922 nm are possible so that the offset of the center wavelength of the band pass filter is effective to filter soliton pulses or noise at the wavelength of the source solitons.
In yet a further embodiment, an optical amplifier can be optically incorporated into the fiber that carries the signal pulses switched through the optical loop mirror. This amplifier serves to offset losses in the loop mirror and in the band pass filters. The amplifiers can be, for example, lumped or distributed erbium doped optical amplifiers. Also semiconductor optical amplifiers may be used.
The length of the waveguide fiber of the optical loop mirror itself is selected to be effective to allow a desired interaction length between the control and signal pulses. A non-linear optical loop mirror length thus is in the range of about 200 m to 2 km. For signal pulses in the wavelength range around 1550 nm, a dispersion shifted fiber, a fiber having a dispersion zero in the range of about 1400 nm to 1600 nm, is used in the NOLM. Longer and shorter optical loop mirror lengths have been used by those skilled in the art. These longer or shorter lengths could be incorporated into the optical loop mirror of the present invention.
In a further embodiment of the invention, the second waveguide fiber (or the first waveguide fiber) can include a variable time delay component so that the relative lag or lead time between signal and control pulses may be adjusted.
Additional embodiments of the invention include those that have different ratios of coupling to the output ports. For example, the first coupler can divide the power equally between the CW and CCW pulses propagating in the optical loop mirror. Advantageous configurations include those having the coupling ratios of the first coupler in the range of about 30%::70% to 70%::30%. Likewise the power ratios of the splitting coupler outputs can be varied between about 10%::90% to 90%::10%.
In a particular embodiment of the invention, the splitting coupler provides about 10% of the input soliton power to the tap coupler. This configuration provides-for higher power signal pulses exiting the optical loop mirror. An optical amplifier can be added into the second fiber, which optically joins the splitting coupler to the tap coupler, to maintain the control pulse amplitude at a a level sufficient to provide for non-linear cross phase modulation of the co-propagating signal pulse by the control pulse.
The optical amplifier in this second connecting fiber can be a lumped or distributed fiber amplifier or a semiconductor optical amplifier. In this latter case, the semiconductor optical amplifier can be selected to shift the wavelength of the control pulses. The tap couplers that respectively couple the control pulses into and out of the loop mirror can then be chosen to be wavelength selective. This is yet another alternative to the use of polarization selective tap couplers.
The optical circuit described above can be used in long transmission links designed for soliton signals. Thus an additional aspect of the invention is an optical waveguide transmission circuit that incorporates one or more of the optical circuits including a NOLM described above. The incorporation of one or more of these optical circuits into a transmission circuit serves to maintain the desired shape and signal to noise ratio of the signal solitons without recourse to electronic regeneration of the signal.
In an embodiment of the optical waveguide transmission circuit, an optical loop mirror circuit is inserted after the solitons have traversed between 25 km to 50 km of waveguide fiber. Signals exiting this first optical circuit can traverse about 50 km to 75 km of the transmission circuit before another optical loop mirror is needed to again reshape the soliton signal pulses.
In a further embodiment of the optical waveguide transmission circuit, a number of optical amplifiers or band pass filters may be optically incorporated into the extended fiber lengths before or after the optical loop mirror circuits.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.